Solid electrolyte capacitors useful in mobile communication devices generally comprise an electrically conductive carrier of high specific surface, covered by a non-conductive niobium or tantalum pentoxide layer taking advantage from the high stability and high dielectric constant of the valve metal oxide, wherein the isolating pentoxide layer can be generated by electrolytic oxidation at very constant thickness. The valve metal or conductive lower oxides (suboxides, NbOx) of the valve metals are used as the carrier material. The carrier, which forms one of the electrodes (anode) of the capacitor generally has a highly porous sponge-like structure which is generated by sintering of very fine primary structures or sponge-like secondary structures. The surface of the conductive carrier structure is electrolytically oxidized (“forming”), whereby the thickness of the isolating pentoxide layer is determined by the maximum voltage of the electrolytic oxidation (“forming voltage”). The counter electrode is generated by soaking of the sponge-like surface-oxidized structure with manganese nitrate, which is thermally transformed into manganese dioxide, or, by soaking of a liquid precursor of a polymer electrolyte (e.g. PEDT, polypyrole) and polymerisation thereof. Electrical terminals are a tantalum or niobium wire sintered with the sponge-like structure at the anode side and the metallic housing of the capacitor, which is isolated against the wire at the cathode side.
The capacitance C of the capacitor is calculated according to the formulaC=(F·∈)/(d·VF),wherein F is the active surface of the capacitor, ∈ is the dielectric constant of the pentoxide layer, d is the thickness of the isolating pentoxide layer per Volt forming voltage, and VF is the forming voltage. The ratio ∈/d is nearly equal for tantalum pentoxide and niobium pentoxide (1.64 resp. 1.69), although ∈ (27.6 resp. 41) and d (16.6 resp. 25 A/V) differ appreciably. Accordingly, capacitors on basis of both the pentoxides having the same geometrical structure have the same capacitance. Specific capacitances per weight differ due to the different densities of Nb, NbOx and Ta respectively. Carrier (anode) structures of Nb or NbOx, accordingly, do have the advantage of saving weight, when used in mobile phones, where reduction of weight is one of the objects. Regarding costs, NbOx is more feasible than Nb, providing part of the volume of the anode structure from oxygen.
An important quality criterion is life time of the capacitor, which depends from the voltage of operation thereof and decreases with increasing voltage. For opening up a wider range of applications, it would be desirable to increase the lifetime, particularly in the upper voltage of operation level.
Furthermore it would be desirable to allow for an increase of the temperature of operation. Presently, the temperature of operation of capacitors based on NbO is limited to about 125° C. A higher allowable temperature of operation would open up the use of capacitors on basis of NbO in the automotive industry.
Furthermore, with reference to safety aspects, it would be desirable to increase the breakdown voltage, and to slow down the burning rate, and to reduce the generation of heat during burning after ignition, of the powders, the sintered anode structures and of the capacitors.